Method of Providing a Zirconium Surface and Resulting Product

ABSTRACT

A coating of blue-black or black zirconium of uniform and controlled thickness on a zirconium or zirconium alloy material is accomplished through the treatment of an amorphous zirconium or zirconium alloy substrate, which may have an altered surface roughness. The treatment of amorphous zirconium or zirconium alloy substrates includes oxidation of the substrates. A zirconium coating of uniform and controlled thickness is especially useful in various applications because the uniformly thick zirconium surface of controlled depth provide a barrier against implant corrosion caused by ionization of the metal substrates.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S.application Ser. No. 12/971,933 filed on Dec. 17, 2010, which is acontinuation application of U.S. application Ser. No. 12/277,013 filedon Nov. 24, 2008, now U.S. Pat. No. 7,896,926, which is a division ofU.S. application Ser. No. 10/942,464 filed on Sep. 16, 2004, now U.S.Pat. No. 7,473,278, the disclosures of which are incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION

This invention relates to metallic implants with load bearing surfacescoated with a thin, dense, low friction, highly wear-resistant,uniformly thick coating of oxidized zirconium.

The invention also relates to uniformly thick oxidized zirconiumcoatings on the non-load bearing surfaces of an orthopedic implant wherethe oxidized zirconium provides a barrier between the metallicprosthesis and body tissue, thereby preventing the release of metal ionsand corrosion of the implant. The invention further relates to a methodof producing a uniformly thick oxidized zirconium layer by using anamorphous zirconium or zirconium alloy substrate with an altered surfaceroughness prior to formation of the oxide layer.

The excellent corrosion resistance of zirconium has been known for manyyears. Zirconium displays excellent corrosion resistance in many aqueousand non-aqueous media, and for this reason has seen an increased use inthe chemical process industry and in medical applications. A limitationto the wide application of zirconium in these areas is its relativelylow resistance to abrasion and its tendency to gall. This relatively lowresistance to abrasion and the tendency to gall is also demonstrated inzirconium alloys.

Orthopaedic implant materials must combine high strength, corrosionresistance and tissue compatibility. The longevity of the implant is ofprime importance, especially if the recipient of the implant isrelatively young because it is desirable that the implant function forthe complete lifetime of a patient. Of the conventional materialstypically used to fabricate orthopaedic implants, each has itscomparative advantages and disadvantages. In the case of metallicmaterials, because certain metal alloys have the required mechanicalstrength and biocompatibility without a high risk of brittle fracture,they are ideal candidates for the fabrication of prostheses. Thesealloys include 316L stainless steel, chrome-cobalt-molybdenum alloysand, more recently, titanium alloys which have proven to be the mostsuitable materials for the fabrication of load-bearing prostheses.However, metallic materials also have disadvantages. They are often notcompletely inert in the body. Body fluids act upon the metals, causingthem to slowly corrode by an ionizing process that thereby releasesmetal ions into the body. Metal ion release from the prosthesis is alsorelated to the rate of wear of load bearing surfaces because the passiveoxide film, which is formed on the surface, is constantly removed. Therepassivation process constantly releases metal ions during the ionizingprocess. Furthermore, the presence of third-body wear (cement or bonedebris) accelerates this process and microfretted metal particlesincrease friction against opposing surfaces. Surface hardness is notideal in most metallic materials, resulting in scratching andmicrofretting. Other common materials have other advantages anddisadvantages. Ceramics, for examples, have very hard surfaces whichresist scratching and microfretting. However, they are more brittle thanmetals and generally have inferior thermal properties.

In addition, the application of a ceramic coating to metal substratesoften results in non-uniform, poorly adhering coatings which tend tocrack due to the differences in elastic modulus or thermal expansionbetween the ceramic and underlying metal substrate. Furthermore, suchcoatings tend to be relatively thick (50-300 microns) and since the bondbetween the metal and the ceramic coating is often weak, there is therisk of galling or separation of ceramic coatings.

The use of oxidized zirconium surfaces in orthopaedic implantsrepresented an advance in that it allowed one to realize the advantagesof metallic materials and ceramics while minimizing the disadvantages ofboth. Previous attempts have been made to produce oxidized zirconiumcoatings on zirconium parts for the purpose of increasing their abrasionresistance. One such process is disclosed in U.S. Pat. No. 3,615,885 toWatson which discloses a procedure for developing thick (up to 0.23 mm)oxide layers on various zirconium alloys. However, this procedureresults in significant dimensional changes, especially for parts havinga thickness below about 5 mm, and the oxide film produced does notexhibit especially high abrasion resistance.

U.S. Pat. No. 2,987,352 to Watson discloses a method of producing ablue-black oxide coating on zirconium alloy parts for the purpose ofincreasing their abrasion resistance. Both U.S. Pat. No. 2,987,352 andU.S. Pat. No. 3,615,885 produce an oxidized zirconium coating onzirconium alloy by means of air oxidation. U.S. Pat. No. 3,615,885continues the air oxidation long enough to produce a beige coating ofgreater thickness than the blue-black coating of U.S. Pat. No.2,987,352. This beige coating does not have the wear resistance of theblue-black coating and is thus not applicable to many parts where thereare two work faces in close proximity. The beige coating wears down morequickly than the blue-black oxide coating with the resulting formationof oxidized zirconium particles and the loss of the integrity of theoxidized zirconium surface. With the loss of the oxide surface, thezirconium metal is then exposed to its environment and can lead totransport of zirconium joints away from the surface of the metal intothe adjacent environment. U.S. Pat. Nos. 2,987,352 and 3,615,885 areincorporated by reference as though fully disclosed herein.

The blue-black coatings have a thickness which is less than that of thebeige coating although the hardness of the blue-black or black coatingis higher than that of the beige coating. This harder blue-black oxidecoating lends itself better to surfaces such as prosthetic devices.Although the blue-black or black coating is more abrasion resistant thanthe beige coating, it is a relatively thin coating. It is thereforedesirable to produce blue-black coatings of increased abrasionresistance without producing the same type coatings of the prior art.

U.S. Pat. No. 5,037,438 to Davidson discloses a method of producingzirconium alloy prostheses with a oxidized zirconium surface. Thisspecific form of oxidized zirconium, described therein as blue-black orblack oxidized zirconium, is unique with respect to the excellentthermal conductivity it possesses relative to other conventionalprosthetic materials. It combines excellent surface roughnesscharacteristics with very high thermal conductivity. In this way, itpossesses the relevant beneficial characteristics of metal and ceramicswhile avoiding the relevant disadvantages of the former andoutperforming the latter. U.S. Pat. No. 5,037,438 is incorporated byreference as though fully set forth herein.

While the introduction of oxidized zirconium for medical implantsrepresented an advance in this area, there was and is room for furtherrefinements. While the durability of the oxidized zirconium implants isexcellent compared to conventional materials, it has been recognizedthat high-integrity oxidized zirconium surfaces have even betterdurability. A high-integrity oxidized zirconium surface can be producedif the thickness of the oxidized zirconium layer has good uniformity.Non-uniformity of thickness negatively affects durability because itpromotes the build-up of internal stresses in the oxide layer and suchstresses tend to lead to cracks. Early on in the use of these surfacesfor medical implants, uniformity of thickness was not controlled. It hasbeen recognized that controlling uniformity of thickness results in abetter oxidized zirconium surface for medical implant applications.

In U.S. Pat. No. 6,447,550, Hunter, et al. described a method forobtaining an oxidized zirconium coating of uniform thickness. Huntertaught that such is obtained by applying pre-oxidation treatmenttechniques to various zirconium-based materials that result in a refinedmicrostructure and an altered surface roughness. Microstructurerefinement is taught in the '550 patent by techniques which include thehot forge conversion of ingot to wrought barstock, closed die forging,rapid solidification, and powder consolidation. The altered surfaceroughness is accomplished by processes such as grinding, buffing, massfinishing, vibratory finishing, among others. U.S. Pat. No. 6,447,550 isincorporated by reference as though fully set forth herein.

Hunter, et al., in U.S. Pat. No. 6,585,772, provide another method forforming a uniformly thick oxide coating on zirconium or a zirconiumalloy. In the method of the '772 patent, Hunter teaches that by inducingan altered surface roughness on a single phase/single compositionzirconium based substrate prior to oxidation, control and improvement ofthe thickness uniformity of the resulting oxidized zirconium layer canbe realized. The '772 patent also provides a method for forming auniformly thick oxide coating on a zirconium or zirconium alloyprosthesis, for implantation in a patient, by inducing an alteredsurface roughness on at least a portion of the zirconium or zirconiumalloy prosthesis, wherein the zirconium or oxidized zirconium consists,at least in part, of a single phase crystalline structure and uniformcomposition, prior to oxidizing the prosthesis to form a blue-blackoxidized zirconium coating of uniform and controlled thickness on atleast a portion of the surface of the prosthesis. U.S. Pat. No.6,585,772 is incorporated by reference as though fully set forth herein.

While both of these techniques of Hunter proved useful in promotingthickness uniformity, the present invention provides yet anotherdistinct method to accomplish the same result. In the present invention,a uniform thickness layer of oxidized zirconium is accomplished using anamorphous zirconium-containing alloy. This represents another techniqueto produce a surface layer of oxidized zirconium having improveduniformity of thickness and provides another tool in the arsenal of onewishing to fabricate improved medical devices having oxidized zirconiumsurfaces.

SUMMARY OF THE INVENTION

The invention provides a zirconium or zirconium-containing metal alloyprosthesis or implant coated, at least in part, via in situ oxidationwith a uniformly thick blue-black or black layer of oxidized zirconiumand a method of forming the aforementioned uniform coating. The uniformcoating of oxidized zirconium provides the prosthesis with a thin,dense, low friction, wear resistant, biocompatible surface ideallysuited for use on articulating surfaces of joint prostheses wherein asurface or surfaces of the joint articulates, translates, or rotatesagainst mating joint surfaces which are also coated with oxidizedzirconium. The uniform oxidized zirconium coating may therefore beusefully employed on the femoral heads or inside surfaces of acetabularcups of hip-joint implants or on the articulating surfaces of othertypes of prostheses, such as but not limited to knee, shoulder or elbowjoints, spinal implants, bone plates and bone screws, etc.

In one embodiment of the present invention, there is a method ofproducing a uniform coating of blue-black or black oxidized zirconium ona zirconium or zirconium alloy, characterized by the step of oxidizingzirconium having an amorphous structure or a zirconium alloy having anamorphous structure. In some embodiments, the method further comprisesthe step of altering the surface roughness of said zirconium orzirconium alloy prior to said step of oxidizing. In some embodiments,the step of altering said surface roughness comprises altering to asurface roughness (Ra) in the range of about 3 microinches to about 25microinches. In some embodiments, the step of altering said surfaceroughness comprises altering to a surface roughness (Ra) in the range ofabout 3.5 microinches to about 7 microinches. In some embodiments, thestep of altering said surface roughness comprises an abrasive surfacepreparation process comprising a step selected from the group consistingof grinding, buffing, mass finishing and vibratory finishing and anycombination thereof. In some embodiments, the step of oxidizingcomprises the use of air as an oxidant. In some embodiments, the step ofoxidizing comprises the use of oxygen as an oxidant. In someembodiments, the zirconium or zirconium alloy comprises about 0.3percent oxygen by weight.

In another embodiment of the present invention, there is a medicalimplant characterized by at least one component, said componentcomprising a substrate comprising zirconium having an amorphousstructure or a zirconium alloy having an amorphous structure; a surfacelayer of blue-black or black oxidized zirconium on at least a portion ofsaid substrate, said surface layer of oxidized zirconium being formed byoxidation of said zirconium or zirconium alloy. In some embodiments, thezirconium or zirconium alloy comprises an altered surface roughnessprior to said oxidation. In some embodiments, the altered surfaceroughness comprises a surface roughness (Ra) in the range of about 3microinches to about 25 microinches. In some embodiments, the alteredsurface roughness comprises a surface roughness (Ra) in the range ofabout 3.5 microinches to about 7 microinches. In some embodiments, thelayer of oxidized zirconium is of a thickness of up to about 20 microns.In some embodiments, the layer of oxidized zirconium coating is of athickness of up to about 10 microns. In some embodiments, the implantportion of the prosthesis body further comprises an irregular surfaceadapted to accommodate tissue ingrowth on a portion of the prosthesisbody. In some embodiments having an irregular surface, the irregularsurface is formed of zirconium or zirconium alloy beads attached to theouter surface of the prosthesis body, wherein at least a portion of thesurface of the beads is oxidized to blue-black or black oxidizedzirconium. In some embodiments having an irregular surface, theirregular surface structure is formed of zirconium or zirconium alloywire mesh connected to the outer surface of the prosthesis body, whereinat least a portion of the surface of the mesh is oxidized to blue-blackor black oxidized zirconium. In some embodiments having an irregularsurface, the irregular surface is a chemically etched surface. In someembodiments having an irregular surface, the irregular surface is plasmaspray-deposited surface. In some embodiments having an irregularsurface, the irregular surface is a sintered surface. In someembodiments, the medical implant is a vertebral implant. In someembodiments, the medical implant is a dental implant. In someembodiments, the medical implant comprises bone implant hardware. Insome embodiments wherein the medical implant comprises bone implanthardware, the bone implant hardware comprises a bone plate or a bonescrew.

In some embodiments of the medical implant having at least onecomponent, the medical implant comprises a first component having abearing surface; and a second component having a counter-bearing surfaceadapted to cooperate with the bearing surface of said first component;wherein at least one of said first component or said second componentcomprises zirconium having an amorphous structure or a zirconium alloyhaving an amorphous structure; a surface layer of blue-black or blackoxidized zirconium on at least a portion of said first component or saidsecond component or both said first and second components, said surfacelayer of oxidized zirconium being formed by oxidation of said zirconiumor zirconium alloy. In some embodiments, the medical implant is a jointprosthesis. In some embodiments having a first component and a secondcomponent, the first component comprises a femoral component and saidsecond component comprises an acetabular cup component to form a hipprosthesis. In some embodiments having a first component and a secondcomponent, the first component comprises a femoral component whichfurther comprises at least one condyle and said second componentcomprises a tibial component to form a knee prosthesis. In someembodiments wherein the medical implant is a joint prosthesis, the jointprosthesis is selected from the group consisting of shoulder, ankle,finger, wrist, toe, or elbow prostheses. In some embodiments, themedical implant is a maxillofacial or temporomandibular implant.

In another aspect of the invention, there is provided a substratecomprising zirconium or a zirconium alloy having an amorphous structure;and a surface layer of blue-black or black zirconium on at least aportion of said zirconium or a zirconium alloy. In some embodiments, aportion of the zirconium or zirconium alloy further comprises an alteredsurface roughness. In other embodiments, the altered surface roughnesscomprises a surface roughness (Ra) in the range of about 3 microinchesto about 25 microinches, or in other embodiments, the range is about 3.5microinches to about 7 microinches. In some embodiments, the surfacelayer has a thickness of up to about 20 microns, or in otherembodiments, the thickness is up to about 10 microns.

In yet some embodiments, a portion of the surface layer furthercomprises an irregular surface. In some embodiments, the irregularsurface comprises zirconium or zirconium alloy beads attached to theouter surface of the substrate, and wherein at least a portion of thesurface of the beads is oxidized to blue-black or black oxidizedzirconium. In other embodiments, the irregular surface compriseszirconium or zirconium alloy wire mesh connected to the outer surface ofthe substrate, wherein at least a portion of the surface of the mesh isoxidized to blue-black or black oxidized zirconium. In yet otherembodiments, the irregular surface comprises a chemically etchedsurface. In other embodiments, the irregular surface comprises a plasmaspray-deposited surface. In some other embodiments, the irregularsurface comprises a sintered surface.

In some embodiments, the surface layer has a substantially uniformthickness. In some embodiments, the surface layer of zirconium has athickness in the range of about 3 microns to about 7 microns. In someembodiments, the surface layer of zirconium comprises a polishedsurface. In other embodiments, the surface layer comprises oxidizedzirconium. In some embodiments, the oxidized surface layer is formed byoxidation of said zirconium or zirconium alloy. In some embodiments, theoxidation of the zirconium or zirconium alloy comprises furnaceoxidation with oxygen at a temperature in the range of about 900 degreesF. to 1300 degrees F. In other embodiments, the oxidation of thezirconium or zirconium alloy comprises salt-bath oxidation with at leastone salt selected from the group consisting of chlorides, nitrates,cyanides, and a combination thereof. In some embodiments, a portion ofsaid zirconium or zirconium alloy further comprises an altered surfaceroughness prior to said oxidation. In yet another embodiment, thealtered surface roughness is formed by subjecting the portion ofzirconium or zirconium alloy to an abrasive surface preparation processselected from the group consisting of grinding, buffing, mass finishing,vibratory finishing, and a combination thereof.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated that the conception and specific embodimentdisclosed may be readily utilized as a basis for modifying or designingother structures for carrying out the same purposes of the presentinvention. It should also be realized that such equivalent constructionsdo not depart from the invention as set forth in the appended claims.The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages will be better understood from thefollowing description when considered in connection with theaccompanying figures. It is to be expressly understood, however, thateach of the figures is provided for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe present invention.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram depicting a hip joint prosthesis inposition.

FIG. 2 is a schematic diagram showing a typical hip join prosthesis.

FIG. 3 is a schematic diagram of a knee joint prosthesis in place.

FIG. 4 is a schematic diagram of the parts of a typical knee joint.

FIG. 5 shows a sample of zirconium alloy mounted on Bakelite afteroxidation for 1 hour at 630° C.

FIG. 6 shows a sample mounted on Bakelite after oxidation for 3 hours at630° C.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, “a” or “an” may mean one or more. As used herein in theclaim(s), when used in conjunction with the word “comprising”, the words“a” or “an” may mean one or more than one. As used herein, “another” maymean at least a second or more.

As used herein, the term “amorphous” or “amorphous structure” means thecondition of lacking long range crystalline order.

As used herein, “zirconium alloy” is defined as any metal alloycontaining zirconium in any amount greater than zero. Thus, an alloy inwhich zirconium is a minor constituent is considered a “zirconium alloy”herein.

The following discussion contains illustrations and examples ofpreferred embodiments for practicing the present invention. However,they are not limiting examples. Other examples and methods are possiblein practicing the present invention.

One aspect of the present invention is to provide a method for formingan oxide coating of uniform thickness on zirconium or a zirconium alloy,using a zirconium which is amorphous or a zirconium alloy which isamorphous. Accordingly, as used throughout herein, the expression“zirconium or zirconium alloy having an amorphous structure” issynonymous with the expression “zirconium having an amorphous structureor a zirconium alloy having an amorphous structure.” In a preferredembodiment, an altered surface roughness is induced on the amorphouszirconium or zirconium alloy prior to oxidation. Another aspect of thepresent invention is to provide a low friction, wear resistant oxidecoating of uniform thickness on prosthesis surfaces, such asarticulating surfaces and irregular surface structures adapted toaccommodate tissue ingrowth on a portion of the prosthesis body.

The subject method of forming an oxide coating of uniform thicknesscomprises oxidizing an amorphous zirconium or an amorphous zirconiumalloy. Preferably, the method also comprises the step of altering thesurface roughness of the amorphous zirconium or zirconium alloy prior tothe step of oxidizing. The resulting oxidized zirconium is applicable toa variety of medical implants, prosthetic parts, and devices. Theseprosthetic parts and devices include, but are not limited to,cardiovascular implants including heart valves, total artificial heartimplants, ventricular assist devices, vascular grafts and stents;electrical signal carrying devices such as pacemaker and neurologicalleads, and defibrillator leads; guide wires and catheters; percutaneousdevices; and joint prostheses including hip joints or surfacereplacements, knee joints, shoulder joints, elbows, endoprostheses,spinal segments, and fingers. Illustrative examples of such articulatingsurfaces are shown in the schematic diagrams of FIGS. 1-4. Additionally,applications are possible in non-articulating implant devices such asbone plates, bone screws, etc.

A typical hip joint assembly is shown in situ in FIG. 1. FIG. 2illustrates a typical hip prosthesis prior to implantation, providingdetails of the individual components. The hip joint stem 2 fits into thefemur while the femoral head 6 of the prosthesis fits into andarticulates against the inner lining 8 of an acetabular cup 10 which inturn is affixed to the pelvis as shown in FIG. 1. A porous metal bead orwire mesh coating 12 may be incorporated to allow stabilization of theimplant by ingrowth of surrounding tissue into the porous coating.Similarly, such a porous metal bead or wire mesh coating can also beapplied to the acetabular component. The femoral head 6 may be anintegral part of the hip joint stem 2 or may be a separate componentmounted upon a conical taper at the end of the neck 4 of the hip jointprosthesis. This allows the fabrication of a prosthesis having ametallic stem and neck, but having a femoral head of some othermaterial, such as ceramic. This method of construction is oftendesirable because ceramics have been found to generate less frictionaltorque and wear when articulating against UHMWPE, which is the typicallining of an acetabular cup. Additionally, zirconia ceramic has beenshown to produce less wear of the UHMWPE than alumina. Regardless of thematerials, however, the femoral head articulates against the innersurface of the acetabular cup, thereby causing wear and, in the longterm, this may necessitate prosthesis replacement. This is especiallythe case where the femoral head is of metal and the acetabular cup islined with an organic polymer or composite thereof. While thesepolymeric surfaces provide good, relatively low friction surfaces andare biocompatible, they are subject to wear and accelerated creep due tothe frictional heat and torque to which they are subjected duringordinary use.

While UHMWPE that has been cross-linked through irradiation followed bya heating step has been shown to exhibit greater wear resistance, it hassimilar shortcomings. A typical knee joint prosthesis is shown in situin FIG. 3. FIG. 4 illustrates a typical knee prosthesis prior toimplantation, providing details of the individual components. The kneejoint includes a femoral component 20 and a tibial component 30. Thefemoral component includes condyles 22 which provide the articulatingsurface of the femoral component, and pegs 24 for affixing the femoralcomponent to the femur. The tibial component 30 includes a tibial base32 with a peg 34 for mounting the tibial base onto the tibia. A tibiaplatform 36 is mounted atop the tibial base 32 and is supplied withgrooves 38 similar to the shape of the condyles 22. The bottom surfacesof the condyles 26 contact the tibial platform's grooves 38 so that thecondyles articulate within these grooves against the tibial platform.While condyles are typically fabricated of metals, the tibial platformmay be made from an organic polymer or a polymer-based composite. Thus,the hard metallic condyle surfaces 26 would articulate against arelatively softer organic composition. This may result in wear of theorganic material, i.e. the tibial platform, necessitating thereplacement of the prosthesis. As in the case of the hip joint, porousbead or wire mesh coatings can also be applied to either the tibial orfemoral components of the knee, or both.

The invention provides uniformly thick oxidized zirconium coatedorthopedic implants or prostheses fabricated of zirconium or zirconiumcontaining metal alloys or a thin coating of zirconium or zirconiumalloy on conventional orthopedic implant materials. Uniformity ofthickness is desirable to insure that the characteristics and propertiesof the layer of oxidized zirconium are relatively constant over thesurface of the prosthetic device. In order to form continuous and usefuloxidized zirconium coatings of uniform thickness over the desiredsurface of the metal alloy prosthesis substrate, the metal alloypreferably contains from about 80 to about 100 wt % zirconium, and morepreferably from about 94 to about 100 wt %. Oxygen and other commonalloying elements may be used in the alloy. For example, in onepreferred embodiment, the zirconium or zirconium alloy is about 0.3% byweight of oxygen.

Although not wishing to be bound to theory, it is believed that anamorphous zirconium or zirconium alloy is useful in the presentinvention because the lack of crystallinity necessarily eliminates grainboundaries in the material. The simplest repeating unit in a crystallinematerial is the unit cell. Unit cell crystals form colonies (dendrites).As solidification nears completion, the dendrites contact one another.These area of contacts are grain boundaries. In the oxidative processthat forms the oxidized zirconium coating, the rate of oxidation ismodified at grain boundaries relative to the rate in the bulk of thematerial. Eliminating or minimizing grain boundaries thereby minimizeschanneling of oxidation through grain boundaries, resulting innon-uniformity of thickness of the resulting layer of oxidizedzirconium. Optionally, altering the surface roughness of the amorphouszirconium or zirconium alloy serves to further enhance thicknessuniformity and, as a result, oxide integrity. Again not wishing to bebound by theory, it is believed that roughening the surface to thevalues described herein increases the number of initiation sites foroxidation, resulting in uniform growth of the oxidation layer inwardtoward the substrate.

The base amorphous zirconium or zirconium containing metal alloys arefabricated by conventional methods to the shape and size desired toobtain a prosthesis substrate. In the preferred embodiment, thesubstrate amorphous zirconium or zirconium alloy is subjected to anabrasive surface preparation process prior to oxidation that includes,but is not limited to, grinding, buffing, mass finishing and vibratoryfinishing. The abrasive surface preparation process is used to induce analtered surface roughness (Ra) of from about 3 microinches to about 25microinches. Alternatively, the range of surface roughness can be fromabout 3.5 to about 7 microinches. The appropriate altered surfaceroughness is induced by altering the pre-existing surface roughness toan altered surface roughness of such a magnitude as to permit theformation of a uniform oxide coating when the amorphous zirconium orzirconium alloy is subjected to the prescribed oxidation process.

The substrate is then subjected to process conditions which cause thenatural (in situ) formation of a tightly adhered, diffusion-bondedcoating of uniformly thick oxidized zirconium on its surface. Theprocess conditions include, for instance, air, steam, or water oxidationor oxidation in a salt bath. These processes ideally provide a thin,hard, dense, blue-black or black, low-friction, wear-resistant,uniformly thick, oxidized zirconium film or coating of thicknessestypically on the order of several microns on the surface of theprosthesis substrate. Below this coating, diffused oxygen from theoxidation process increases the hardness and strength of the underlyingsubstrate metal.

The air, steam and water oxidation processes are described innow-expired U.S. Pat. No. 2,987,352 to Watson, the teachings of whichare incorporated by reference as though fully set forth herein. Theoxidation process applied to an amorphous zirconium or zirconium alloy,preferably having an altered surface roughness, provides a firmlyadherent black or blue-black layer of uniformly thick oxidizedzirconium. If the oxidation is continued to excess, the coating willwhiten and separate from the metal substrate. For convenience, the metalprosthesis substrate may be placed in a furnace having anoxygen-containing atmosphere (such as air) and typically heated at900°-1300° F. for up to about 6 hours. However, other combinations oftemperature and time are possible. When higher temperatures areemployed, the oxidation time should be reduced to avoid the formation ofthe white oxide.

One of the salt-bath methods that can be used to apply the oxidizedzirconium coatings to the metal alloy prosthesis is the method of U.S.Pat. No. 4,671,824 to Haygarth, the teachings of which are incorporatedby reference as though fully set forth herein. The salt-bath methodprovides a similar, slightly more abrasion resistant blue-black or blackoxidized zirconium coating. This method requires the presence of anoxidation compound capable of oxidizing zirconium in a molten salt bath.The molten salts include chlorides, nitrates, cyanides, and the like.The oxidation compound, sodium carbonate, is present in smallquantities, up to about 5 wt %. The addition of sodium carbonate lowersthe melting point of the salt. As in air oxidation, the rate ofoxidation is proportional to the temperature of the molten salt bath andthe '824 patent prefers the range of 550°-800° C. (1022°-1470° F.).However, the lower oxygen levels in the bath produce thinner coatingscompared to furnace air oxidation at the same time and temperature. Asalt bath treatment at 1290° F. for four hours produces an oxide coatingthickness of roughly 7 microns.

The overall thickness of the oxidized zirconium coating is primarilycontrolled by the variables of time and temperature of the in-situgrowth process. The instant invention is concerned with the uniformityof thickness of the coating so created. Creation of a uniform oxidecoating during the oxidation process is dependent on both a surface withappropriate altered surface roughness and amorphous composition. Theoxide coating initiates and grows from surface asperities, so the oxideinitiation sites may be spaced too far apart to produce a uniformcoating thickness on a surface that is too smooth. The oxide layer growsby oxygen diffusion along grain boundaries and through microstructuralgrains. The oxidation rate can be different in grains of differentstructure and composition. Thus, the oxide coating may not grow with auniform thickness through a heterogeneous microstructure. Althoughspecific limits for the necessary minimum surface roughness can be alloyand application dependent, sufficient phase homogeneity may be achievedthrough the use of an amorphous zirconium metal or amorphous zirconiumalloy.

The uniformly thick oxidized zirconium coating may range up to about 20microns. It is preferred that a uniformly thick blue-black oxidizedzirconium layer ranging in thickness from about 1 to about 10 micronsshould be formed. It is most preferred that the uniformly thick oxidizedzirconium layer ranges from about 3 microns to about 7 microns. Forexample, furnace air oxidation at 1100° F. for 3 hours will form auniform oxide coating of a thickness of 4-5 microns on a zirconium alloyhaving greater than 96 wt % zirconium with a surface roughness (Ra) ofabout 4 microinches. Longer oxidation times and higher oxidationtemperatures will increase this thickness, but may compromise coatingintegrity. Thicknesses of up to 20 microns or greater can be achievedunder appropriate conditions. For example, one hour at 1300° F. willform an oxide coating thickness of about 9 microns. Of course, becauseonly a thin oxide is necessary on the surface, only very smalldimensional changes, typically less than 10 microns over the thicknessof the prosthesis, will result. In general, thinner coatings (1-10microns) have better attachment strength. However, depending upon theapplication, coatings of greater thickness may be used.

Blue-black or black oxidized zirconium coatings produced by any of theprior art methods are quite similar in hardness. For example, if thesurface of a wrought zirconium alloy prosthesis substrate is oxidized,the hardness of the surface shows a dramatic increase over the 200 Knoophardness of the original metal surface. The surface hardness of theblue-black oxidized zirconium surface following oxidation by either saltbath or air oxidation process is approximately 1200-1700 Knoop hardness.

The diffusion-bonded, low friction, highly wear resistant, uniformlythick oxidized zirconium coatings of the present invention can beapplied to the surfaces of orthopedic implants subject to conditions ofwear, and to prosthetic implants and devices requiring a biocompatiblesurface. Such surfaces include, but are not limited to, the articulatingsurfaces of knee joints, elbows and hip joints. In the case of hipprostheses (FIGS. 1 and 2), the femoral head 6 is an example of wherethe oxidized zirconium coating may be located. In such prostheses, thefemoral head and stem are typically fabricated of metal alloys, whilethe acetabular cup may be fabricated from ceramics, metals or organicpolymer-lined metals or ceramics. However, any other portions of theprostheses may have the oxidized zirconium coating of the presentinvention. In the case of knee prostheses (FIGS. 3 and 4), the condylesurface 26 is an example of where the oxidized zirconium coating may belocated. However, any other portions of the prostheses may have theoxidized zirconium coating of the present invention.

When the oxidized zirconium coatings are applied to surfaces subject towear, it is desirable to obtain a smooth finished surface to minimizeabrasive wear. After the oxidation process, the oxide coating surfacecan be polished by any of a variety of conventional finishingtechniques. Sufficient oxide thickness must be produced to accommodatethe chosen finishing technique. For example, a surface with a uniformoxide coating of about 5 microns thick and having a pre-oxidationsurface roughness (Ra) of about 4 microinches can be burnished to afinal surface roughness (Ra) of about 2 microinches with a loss of about1 micron in oxide thickness.

In the medical implants fabricated using the invention described herein,it is sometimes desirable to have a textured surface to promote bonein-growth and on-growth. A number of techniques known to those ofordinary skill in the art may be combined with the teachings of thepresent invention to fabricate such medical implants. Zirconium orzirconium alloy can also be used to provide a porous bead or wire meshsurface to which surrounding bone or other tissue may integrate tostabilize the prosthesis. These porous coatings can be treatedsimultaneously by the oxidation of the base prosthesis for theelimination or reduction of metal ion release. Furthermore, zirconium orzirconium alloy can also be used as a surface layer applied overconventional implant materials prior to inducing an altered surfaceroughness, in situ oxidation, and formation of the uniform oxidizedzirconium coating. Other applicable means to achieve a textured surface,also known to those of ordinary skill in the art, include chemicaletching, various deposition methods such as chemical vapor deposition,plasma-spray deposition, etc., as well as others.

The process of the present invention provides another avenue to avoidthe problems of the formation of thick oxide coatings of low abrasionresistance and of significant dimensional changes associated with theprocess disclosed in U.S. Pat. No. 3,615,885. The control of bothoverall coating thickness and the uniformity of the coating thicknessaffords a great deal of dimensional control in the fabrication ofprosthetic devices wherein exacting tolerances are required. The presentinvention also produces an oxide film that is highly abrasion resistant,unlike that of the '885 patent.

The process of the present invention, by oxidizing an amorphouszirconium or zirconium alloy, preferably following a step of inducing analtered surface roughness on the amorphous zirconium or zirconium alloy,results in the formation of a blue-black oxidized zirconium coating ofuniform thickness, the depth of which can be controlled by the properchoice of the oxidation conditions. The formation of a uniformly thickoxide coating provides an oxide coating of variable and controlledthickness with especially high abrasion resistance and reduced wear dueto high integrity of the adhesion between the oxide layer and theunderlying zirconium or zirconium alloy, and the high integrity of theadhesion within the oxide layer. The term “high integrity” denotes anoxide coating that is uniform in thickness with no visible cracks orpores when viewed in cross-section by optical microscopy.

The invention provides an amorphous zirconium or zirconium-containingmetal alloy prosthesis coated via in situ oxidation with a coating ofoxidized zirconium of uniform thickness. The uniformly thick oxidizedzirconium coating provides the prosthesis with a thin, dense, lowfriction, high integrity, wear resistant biocompatible surface ideallysuited for use on articulating surfaces of joint prosthesis wherein asurface or surfaces of the joint articulates, translates or rotatesagainst mating joint surfaces. The uniformly thick oxidized zirconiumcoating may therefore be usefully employed in joint prostheses such ason the femoral heads or inside surfaces of acetabular cups of hip-jointimplants, or on the articulating surfaces of other types of prostheses,such as knee joints. Generally, such implants can be described asmulti-component prostheses comprising a first component having a bearingsurface and a second component having a counter-bearing surface, whereat least one of the components comprises zirconium or a zirconium alloyhaving an amorphous structure and where a surface layer of blue-black orblack oxidized zirconium exists on at least a portion of a componentcomprising zirconium or a zirconium alloy, the surface layer of oxidizedzirconium being formed by oxidation of said zirconium or zirconiumalloy. Alternatively, other implants amenable to the present inventionare those having at least one component having a substrate comprisingamorphous zirconium or an amorphous zirconium alloy; a surface layer ofblue-black or black oxidized zirconium on at least a portion of thesubstrate; the surface layer of oxidized zirconium being formed byoxidation of the zirconium or zirconium alloy. The present invention isapplicable to any prostheses, including shoulder, ankle, finger, wrist,toe, elbow, or other prostheses. Other possible applications includemaxillofacial or temporomandibular implants. Other implants such asdental and vertebral implants could be fabricated with uniform thicknessoxidized zirconium surfaces according to the present invention. It isalso applicable to implant hardware, such as bone plates and bonescrews. Other possibilities are clear to one of ordinary skill in theart.

When a joint surface coated with a uniformly thick oxidized zirconium isemployed in a manner wherein it articulates or rotates against anon-metallic or non-oxidized zirconium coated surface, the low frictioncharacteristic and high integrity of the uniformly thick coating causesreduced friction, wear, and heat generation relative to prior artprostheses. This reduced heat generation results in a lowered tendencyfor the non-metallic or non-oxidized zirconium coating bearing surfaceto experience creep and torque so that the useful life of the opposingsurface is enhanced. Organic polymers, such as UHMWPE, exhibit rapidlyincreased rates of creep when subjected to heat with a consequentdeleterious effect on the life span of the liner. Wear debris of thepolymer leads to adverse tissue response and loosening of the device.Thus, not only does the uniformly thick oxidized zirconium coating serveto improve the protection of the prosthesis substrate to which it isapplied due to its high integrity, it also, as a result of its lowfriction surface, protects those surfaces against which it is inoperable contact, and consequently enhances the performance and life ofthe prosthesis.

A uniformly thick oxidized zirconium coated joint surface also enhancesthe useful life of the opposing surface when the opposing surface isbody tissue. The surgical replacement of one component of the joint istermed “hemiarthroplasty”, and because the repaired joint has only oneartificial (prosthesis) component, the artificial component is oftentermed a “unipolar” prosthesis or “endoprosthesis.” The uniformly thickoxidized zirconium coating is a low friction surface for articulation,translation and rotation against body tissue, thereby having the samebeneficial effect for a body tissue counterface as it does for anorganic polymer counterface.

The usefulness of oxidized zirconium coated prosthesis is not limited toload bearing prostheses, especially joints, where a high rate of wearmay be encountered. Other applications are possible in non-articulatingimplant devices such as bone plates, bone screws, etc. Because theuniformly thick oxidized zirconium coating is firmly bonded to thezirconium alloy prosthesis substrate, it provides an enhanced barrierbetween the body fluids and the zirconium alloy metal, therebypreventing the corrosion of the alloy by the process of ionization andits associated metal ion release compared to non-uniform oxide coatings.

Additionally, the natural in situ formation of a uniformly thickoxidized zirconium coating from the presence of zirconium in thesubstrate metal involves oxygen diffusion into the metal substrate belowthe oxide coating. Oxygen, an alloying constituent in zirconium,increases the strength of the metal substrate, particularly the fatiguestrength. A preferred embodiment is one having a zirconium or zirconiumalloy with an oxygen content of about 0.3% by weight. Furthermore, thehigh integrity of the uniformly thick coating reduces the number offatigue crack initiation sites relative to a non-uniformly thick oxidecoating that contains cracks or pores. Resistance to fatigue loading isparamount in many orthopedic implant applications such as the hip stem,and femoral and tibial knee components. Thus, not only does theformation of the uniformly thick oxidized zirconium coating improvewear, friction, and corrosion resistance, it also improves themechanical integrity of the implant device from a strength standpoint.

An amorphous alloy of Zr—Ti—Cu—Ni—Be, with Zr as the major alloyingconstituent (55%), demonstrates the usefulness of the present invention.Samples were oxidized at 630° C. for 1 and 3 hours. The thickness of theoxidized zirconium layer was measured by preparing the sample formetallographic inspection. The metallographic images of the oxidizedsamples are shown in FIGS. 5 (1 hour oxidation) and 6 (3 houroxidation). The average oxide thickness (±standard deviation) after 1hour of oxidation was 1.5±0.4 μm, whereas after 3 hours it was 8.9±0.7μm. FIG. 5 shows a cracked sample of zirconium alloy mounted on Bakeliteafter oxidation for 1 hour at 630° C. FIG. 6 shows a sample mounted onBakelite after oxidation for 3 hours at 630° C. In each case, thezirconium-based amorphous alloy substrate material is shown with anoxidized zirconium layer. The thickness of the layer formed after 1 hourof oxidation had a coefficient of variation of 26.6%, while thethickness of the layer formed after 3 hours of oxidation had acoefficient of variation of 7.9%. The energy dispersive x-ray analysisof the oxide surface showed that it is mainly composed of oxidizedzirconium.

Thus, a 3 hour oxidation using the method of the present invention had acoefficient of variation of about 8%. This result is comparable to themethod of Hunter et al (U.S. Pat. No. 6,447,550) using pre-oxidationtreatment techniques that result in a refined microstructure and analtered surface roughness. An 18% coefficient of variation in theuniformity of thickness of the oxidized zirconium is obtained using themethodology of the '550 patent in which an as-cast material had aroughened surface having an Ra value of 4-8 microinches. When theas-cast material was replaced with a wrought (refined grain size)material, the variation dropped to 6% when the surface was roughened toan Ra value of 4-8 microinches. In comparison, the conventional methodof forming surface layers of oxidized zirconium, as described in U.S.Pat. No. 5,037,438, yields a surface layer thickness with a coefficientof variation of 72%. Thus, the use of the present invention affords anew way to form an oxidized zirconium surface layer having uniformthickness.

Although the invention has been described with reference to itspreferred embodiments, those of ordinary skill in the art may, uponreading this disclosure, appreciate changes and modifications which maybe made and which do not depart from the scope and spirit of theinvention as described above or claimed hereafter.

All patents and publications mentioned in the specification areindicative of the level of those skilled in the art to which theinvention pertains. All patents and publications are herein incorporatedby reference to the same extent as if each individual patent andpublication was specifically and individually indicated to beincorporated by reference.

One skilled in the art readily appreciates that the present invention iswell adapted to carry out the objectives and obtain the ends andadvantages mentioned above as well as those inherent therein. Systems,methods, procedures and techniques described herein are presentlyrepresentative of the preferred embodiments and are intended to beexemplary and are not intended as limitations of scope. Changes andother uses will occur to those skilled in the art which are encompassedwithin the spirit of the invention or defined by the scope of theclaims.

1. A substrate comprising: a zirconium or a zirconium alloy having anamorphous structure; and a surface layer of blue-black or blackzirconium oxide on at least a portion of said zirconium or a zirconiumalloy.
 2. The substrate of claim 1, wherein a portion of said zirconiumor zirconium alloy further comprises an altered surface roughness. 3.The substrate of claim 2, wherein said altered surface roughnesscomprises a surface roughness (Ra) in the a range of about 3 microinchesto about 25 microinches.
 4. The substrate of claim 3, wherein saidaltered surface roughness comprises a surface roughness (Ra) in the arange of about 3.5 microinches to about 7 microinches.
 5. The substrateof claim 1, wherein said surface layer has a thickness of up to about 20microns.
 6. The substrate of claim 1, wherein said surface layer has athickness of up to about 10 microns.
 7. The substrate of claim 1,wherein a portion of the surface layer further comprises an irregularsurface.
 8. The substrate of claim 7, wherein said irregular surfacecomprises zirconium or zirconium alloy beads attached to the an outersurface of the substrate, and wherein at least a portion of the asurface of the beads is oxidized to blue-black or black oxidizedzirconium.
 9. The substrate of claim 7, wherein said irregular surfacecomprises zirconium or zirconium alloy wire mesh connected to an outersurface of the substrate, wherein at least a portion of a surface of themesh is oxidized to blue-black or black oxidized zirconium.
 10. Thesubstrate of claim 7, wherein said irregular surface comprises achemically etched surface.
 11. The substrate of claim 7, wherein saidirregular surface comprises a plasma spray-deposited surface.
 12. Thesubstrate of claim 7, wherein said irregular surface comprises asintered surface.
 13. The substrate of claim 1, wherein said surfacelayer has a substantially uniform thickness.
 14. The substrate of claim1, wherein said surface layer of zirconium oxide has a thickness in arange of about 3 microns to about 7 microns.
 15. The substrate of claim1, wherein said surface layer of zirconium oxide comprises a polishedsurface.
 16. The substrate of claim 1, wherein said surface layercomprises oxidized zirconium.
 17. The substrate of claim 16, whereinsaid oxidized surface layer is formed by oxidation of said zirconium orzirconium alloy
 18. The substrate of claim 17, wherein said oxidation ofthe zirconium or zirconium alloy comprises furnace oxidation with oxygenat a temperature in a range of about 900 degrees F. to 1300 degrees F.19. The substrate of claim 17, wherein said oxidation of the zirconiumor zirconium alloy comprises salt-bath oxidation with at least one saltselected from the group consisting of chlorides, nitrates, cyanides, anda combination thereof.
 20. The substrate of claim 17, wherein a portionof said zirconium or zirconium alloy further comprises an alteredsurface roughness prior to said oxidation.
 21. The substrate of claim20, wherein said altered surface roughness is formed by subjecting saidportion of zirconium or zirconium alloy to an abrasive surfacepreparation process selected from the group consisting of grinding,buffing, mass finishing, vibratory finishing, and a combination thereof.22. A component having a durable, wear-resistant and corrosion-resistantsurface, comprising: a substrate comprising zirconium or a zirconiumalloy having an amorphous structure; and a surface layer of blue-blackor black oxidized zirconium on at least a portion of said substrate,said surface layer of oxidized zirconium formed by oxidation of saidzirconium or zirconium alloy.
 23. The component of claim 22, whereinsaid zirconium or zirconium alloy comprises an altered surface roughnessprior to said oxidation.
 24. The component of claim 23, wherein saidaltered surface roughness comprises a surface roughness (Ra) in therange of about 3 microinches to about 25 microinches.
 25. The componentof claim 23, wherein said altered surface roughness comprises a surfaceroughness (Ra) in the range of about 3.5 microinches to about 7microinches.
 26. The component of claim 22, wherein said surface layerof oxidized zirconium is of a thickness of up to about 20 microns. 27.The component of claim 22, wherein said surface layer of oxidizedzirconium is of a thickness of up to about 10 microns.
 28. A method forproducing a component having a durable, wear-resistant andcorrosion-resistant surface, comprising: providing a substratecomprising zirconium or a zirconium alloy having an amorphous structure;and oxidizing a surface of the substrate under conditions operable toproduce a coating of blue-black or black zirconium oxide on thesubstrate.
 29. The method of claim 28, further comprising, before theoxidizing, altering a surface roughness of the zirconium or zirconiumalloy.
 30. The method of claim 29, wherein the altering comprisesaltering to a surface roughness (Ra) in the range of about 3 microinchesto about 25 microinches.
 31. The method of claim 29, wherein thealtering comprises altering to a surface roughness (Ra) in the range ofabout 3.5 microinches to about 7 microinches.
 32. The method of claim29, wherein the altering the surface roughness comprises an abrasivesurface preparation process comprising an action selected from the groupconsisting of grinding, buffing, mass finishing, vibratory finishing andany combination thereof.
 33. The method of claim 28, wherein theoxidizing comprises the use of air as an oxidant.
 34. The method ofclaim 28, wherein the oxidizing comprises the use of oxygen as anoxidant.
 35. The method of claim 28, wherein the zirconium or zirconiumalloy comprises about 0.3 percent oxygen by weight.
 36. The method ofclaim 28, wherein the oxide coating has an outer surface opposite thesubstrate; and further comprising, after the oxidizing, polishing theouter surface.